Influence of zolpidem, a novel hypnotic, on the intermediate-stage and paradoxical sleep in the rat.

Paradoxical sleep (PS) in mice, rats, and cats is preceded and sometimes followed by a short-lasting stage characterized by cortical high-amplitude spindles and hippocampal low-frequency theta rhythm. This intermediate stage (IS) seems to correspond to a transient functional disconnection of the forebrain from the brainstem. Pentobarbital and benzodiazepines greatly extend IS at the expense of PS, which is suppressed. Zolpidem, a new imidazopyridine hypnotic, was studied at 2.5, 5, and 7.5 mg/kg IP. At 2.5 mg/kg, which is already a true hypnotic dose, it only decreased PS during the first 2 h of recording with a rebound during the following 4 h of recording. At 5 mg/kg, zolpidem increased the number and total duration of IS episodes, increased IS episodes not followed by PS, and increased PS latency of occurrence. PS amount was decreased during the first three h with a rebound in the next 3 h. At 7.5 mg/kg, the total amount of PS was also decreased. The eye movement number and theta rhythm frequency of PS were unchanged. These results show that zolpidem produces less disruption of the association between IS and PS than do previous hypnotics.